An organic electroluminescent device (organic EL device) in the simplest structure is constituted of a light-emitting layer and a pair of counter electrodes sandwiching the said light-emitting layer. The device functions by utilizing the following phenomenon: upon application of an electrical field between the electrodes, electrons injected from the cathode and holes injected from the anode recombine in the light-emitting layer to generate excitons and the energy level of the excitons returns from the conduction band to the valence band with release of energy as light.
Organic EL devices are classified into fluorescent organic EL devices and phosphorescent EL devices by the mechanism of emission of light. The light-emitting layer of a phosphorescent organic EL device is generally composed of a phosphorescent dopant material and a host material. Utilization of a phosphorescent dopant material in emission of light is equivalent to utilization of triplet excitons that are generated with 75% probability; hence, it is likely that a phosphorescent organic EL device is capable of performing at higher luminous efficiency than a fluorescent organic EL device utilizing singlet excitons that are generated with 25% probability.
In recent years, in the field of phosphorescent organic EL devices with a potentiality to attain the aforementioned high luminous efficiency, researches and developmental works are directed toward the use of low-molecular-weight compounds as host materials in the light-emitting layer. One of the main reasons for this trend is that low-molecular-weight compounds to be used as host materials are obtained readily in high purity by such purification techniques as sublimation, column chromatography, and recrystallization. Raising the purity of a low-molecular-weight host material reduces the number of energy trap sites derived from the impurities and minimizes thermal deactivation of electrons and holes injected from the electrodes or of excitons and, as a result, higher purity helps to provide a device of higher performance. On the other hand, low-molecular-weight compounds exhibit the characteristic property common to them that the crystallinity becomes increasingly higher as the purity is raised. In consequence, there may arise a problem that a high-purity low-molecular-weight host material incorporated in the light-emitting layer of a device may partly crystallize in fine crystals by the action of feeble heat generated while the device is in operation and harms the amorphous property of the light-emitting layer to eventually degrade the quality of the device. This special property exerts a profound influence when a film constituting the light-emitting layer is made by a wet process. The plausible explanation is that, in making a film by a wet process, a solution of a low-molecular-weight host compound is applied and the solvent is then evaporated off in the drying step where the solution temporarily becomes highly concentrated or the solution goes through a condition that favors crystallization. This fact presents the largest problem that a low-molecular-weight host material, although it displays high performance in film forming by the vapor deposition process, is not applicable to film forming by a wet process.
To solve the aforementioned problems, a number of methods have been disclosed for securing the stability of the light-emitting layer in the amorphous state by mixing at least another low-molecular-weight host material into the base low-molecular-weight host material. For example, mixing of an amorphous polymeric material is disclosed in patent document 1 and mixing of electrical charge-injecting and electrical charge-transporting auxiliaries is disclosed in patent documents 2-6.